Conventional power tools, such as those used in the assembly of motor vehicles, generally comprise a pneumatic motor connected to a drive shaft that is adapted to carry a tool, such as a hexagonal drive socket for engaging a threaded fastener. Some known power tools incorporate marking means for visibly marking the fastener when the fastener has been tightened to a required torque. This visible indication can then be used for quality control purposes. In one such tool, the marking means comprises a spray device which delivers a quantity of liquid marking material sprayed from a jet onto the workpiece in the region of the fastener when the desired torque has been applied to the fastener. The use of a spray suffers from the disadvantages that the amount of liquid marking medium applied to the workpiece and the position at which the medium is applied are difficult to control. Thus, the tool itself can easily become contaminated with the marking medium, and, if the jet is not aligned correctly with the workpiece, the presence of the marking medium can be difficult to detect.